What resolution to print at 1:6?

[media kat]

I want to take old movie posters that are available in sizes like 995x1500 px, 1946x2944px, 988x1500 px and print them at 1:6. So an old poster may have been 27″ x 40″ so I will need to print it at 4.5 X 6.7".

1. How do I determine how many pixels I need from one of the original images to get a good print?

2. What is a good strategy for resizing these images? Should I make the width 4.5 or make the height 6.7, or does it not really make a difference quality wise?

Thanks

[c.pfaffenbichler]

Just change the dimensions in the File Size dialog to the target size without Resampling. 

What is the resulting resolution? 

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2. What is a good strategy for resizing these images? Should I make the width 4.5 or make the height 6.7, or does it not really make a difference quality wise?

Say what? 

You must make the decision what you want to do about images that do not fit the target proportions – clip them, distort them, keep empty space, …? 

[media kat]

The image I am working with is 27.028 X 40.889 inches though it also shows 1946 px X 2944 px. The resolution is 72.

I change the width to 4.5 inches and the height changes to 6.808, the resolution is now 432.444 and the dimensions at the top still show 1946 px X 2944 px. I did not have the resample checkbox checked.

1. Isn't 1946 px X 2944 px much smaller than the equivalent image measured in inches at 27.028 X 40.889?

2. Why does the pixel amount of 1946 px X 2944 px at the top not change when I changed the width amount to reflect its new size in pixels?

3. I'm assuming that a resolution of 432.444 is very large for printing an image at this size?

Thanks!

[TheDigitalDog]

The image I am working with is 27.028 X 40.889 inches though it also shows 1946 px X 2944 px. The resolution is 72.

Work with pixels! Divide the size you wish into the pixels you have, and ignore the current tag which is rather meaningless. 

On the long axis, you have 2944 pixels. If you divide (ugh) 2944 by the 40 inches you way you want, that's a mere 73.6 PPI which is way, way too low for a print that size. You need probably a minimum of 180 pixels per inch. 40x180 means you need 7200 pixels (maybe more, much depends on the printer and viewing distance). You can resample. Make more pixels out of thin air, but that isn't ideal. 

A primer on resolution, albeit very old seems your first step:

http://digitaldog.net/files/Resolution.pdf

From here, we can move forward with what to do.

[D Fosse]

A raster image is just made of pixels. That's all it is, it has no native size. It's just data points. Zoom in, and this is what it looks like:

Size is determined by the pixels per inch-number (ppi). Say that again: pixels per inch. See the relationship? Read that literally: how many pixels to a physical inch. The ppi number determines the size!

In other words, this is a standard equation. If the ppi number goes up, the size goes down. If the size goes up, the ppi number goes down.

You don't have to perform this calculation yourself (although you could!). Open Image Size and Photoshop will do it for you.

For most small format printing to be seen close up, a pixel density of 300 ppi is recommended. If you have an image with a certain number of pixels, that tells you how big you can print it.

Or if you create a new file, and you know it has to be a certain size at a certain ppi, you know how many pixels you need.

This is all very simple, a simple relationship, but it confuses a lot of people. Probably because it's natural to assume that ppi is a native, built in property of the file. But it's not! Ppi is just a way to translate the given pixels into a physical size, or vice versa.